$this->form_validation->set_rules('username', 'Username', 'required|_max_length[12]');

function _max_length($val)
    {
    	if (strlen($this->input->post('username')) > $val)
    	{
                    $this->form_validation->set_message('_max_length', 'Max characters allowed: 5')
    		return FALSE;
    	}

    	return TRUE;
    }

$this->form_validation->set_message('username_check', 'The %s field can not be the word "test"');

$this->form_validation->set_message('max_length', 'The field %s max length is %s');

$this->form_validation->set_message('validation_rule', 'Your message here');

$this->form_validation->set_message('max_length', 'Max characters allowed: 5');